Dear, Dave It's very interesting to see ASIO and FSM integration (I'd always been confused how to use them together). I haven't seen your code, but you can see mine: https://sourceforge.net/projects/asio-samples/. It's without FSM but with serial port too (Windows, may be Linux/Unix). The main concept is async active object(s) (techniques of their creation and usage on the top of ASIO) with external (user) async operations and internal ones (internal loops of async operatins). Also I tried to heavily use custom memory allocation (to reduce memory allocator calls which slows down multithreaded environment). The code isn't clean enough but it works fine. Best regards, Marat